Bonding Conditions in Diamond

Diamond is an allotrope of carbon. The bonds are of sp type, that is, each carbon is surrounded by four neighboring carbon atoms at equal bond length. The end atoms may be saturated by hydrogen atoms and the system geometry optimized. 

When the hydrogen atoms are replaced by four methyl groups, there are 32 valence MOs, of which 16 have a bonding character. The 32 valence electrons of C(CH3)4 occupy these orbitals and form a structure with a low enthalpy. There is still a large 

Diamond has very high carrier mobility in the conduction band. This is consistent with the fact that, due to delocalization, there is very little reorganization energy. 

The same electronic structure as for diamond is obtained for Si and Ge crystals, but the band gap is considerably smaller 1.17 eV for Si and 0.74 eV for Ge. We replace Ge by its neighbors in the periodic table, to obtain gallium phosphide (GaP), 

FIGURE 16.10 (See color insert) Piece of diamond saturated by hydrogen atoms and with a nitrogen atom (dark grey) replacing a carbon atom (a, left). In (b) we see the HOMO of a spin polarized calculation (center) the LUMO is seen in the right picture (c). Both the HOMO and the LUMO are considerably delocalized. 

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